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51318-11306-Localized corrosion of alloys UNS N06690 and N06600 in steam generator lay-up conditions

Localized corrosion of solubilized and thermally aged alloys UNS N06690 and N06600, used in nuclear power plant steam generator tubing, was studied in simulated wet lay-up conditions.

 

Product Number: 51318-11306-SG
Author: Héctor G. Maristany / Mariano A. Kappes / Martín A. Rodríguez / Martín A. Rodríguez
Publication Date: 2018
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Localized corrosion of solubilized and thermally aged alloys UNS N06690 and N06600, used in nuclear power plant steam generator tubing, was studied in simulated wet lay-up conditions. The results showed that, despite having half of the chromium content of alloy N06690, alloy N06600 was more resistant to crevice corrosion in dilute chloride solutions, at low temperatures. In concentrated chloride solutions, the crevice corrosion resistance of both alloys was similar. The heat treatment used for steam generator tubing, which produces carbide precipitation at grain boundaries, did not reduce the crevice corrosion resistance of tested alloys with respect to solubilized material. Pitting corrosion initiated at higher potentials than crevice corrosion for both alloys. Initiation of pitting corrosion in 0.01 mol/L NaCl, at 30 ºC, occurred at higher potentials for alloy N06690 than for alloy N06600. Initiation potentials of localized corrosion for each alloy were not affected by heat treatment. Open circuit potentials of thermally aged alloys N06600 and N06690 in deaerated 10-4 mol/L LiOH, which simulates lay-up conditions, showed a significant safety margin over crevice corrosion repassivation potentials in 0.01 mol/L NaCl, at 30 ºC. Dissolved oxygen might pose a risk of crevice corrosion in lay-up conditions if chloride concentration build-up in crevices becomes significant.

Key words: N06690, N06600, pitting corrosion

Localized corrosion of solubilized and thermally aged alloys UNS N06690 and N06600, used in nuclear power plant steam generator tubing, was studied in simulated wet lay-up conditions. The results showed that, despite having half of the chromium content of alloy N06690, alloy N06600 was more resistant to crevice corrosion in dilute chloride solutions, at low temperatures. In concentrated chloride solutions, the crevice corrosion resistance of both alloys was similar. The heat treatment used for steam generator tubing, which produces carbide precipitation at grain boundaries, did not reduce the crevice corrosion resistance of tested alloys with respect to solubilized material. Pitting corrosion initiated at higher potentials than crevice corrosion for both alloys. Initiation of pitting corrosion in 0.01 mol/L NaCl, at 30 ºC, occurred at higher potentials for alloy N06690 than for alloy N06600. Initiation potentials of localized corrosion for each alloy were not affected by heat treatment. Open circuit potentials of thermally aged alloys N06600 and N06690 in deaerated 10-4 mol/L LiOH, which simulates lay-up conditions, showed a significant safety margin over crevice corrosion repassivation potentials in 0.01 mol/L NaCl, at 30 ºC. Dissolved oxygen might pose a risk of crevice corrosion in lay-up conditions if chloride concentration build-up in crevices becomes significant.

Key words: N06690, N06600, pitting corrosion

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